aa r X i v : . [ h e p - e x ] J un Searches for New Physics at HERA
E. Sauvan a ∗ a CPPM, IN2P3-CNRS et Universit´e de la M´editerran´ee, 163 Av. de LuminyF-13288 Marseille, France
The high energy programme of the HERA collider ended in March 2007. During the whole HERA programme,a combined total integrated luminosity of 1 fb − was collected by the H1 and ZEUS experiments. In this context,an overview of the most recent results of both experiments concerning searches for new physics is presented. Thetopics covered are searches for contact interactions, leptoquarks and excited leptons, as well as studies of theisolated lepton and multi-lepton topologies, and a general signature based search.
1. Introduction
At HERA electrons (or positrons) collide withprotons at √ s ≃
320 GeV. After two running pe-riods, the high energy data taking ended in March2007. Over the whole HERA running the H1 andZEUS experiments have each recorded ∼ . − of data, shared between e + p and e − p collisionmodes. These high energy electron-proton inter-actions provide a testing ground for the StandardModel (SM) complementary to e + e − and p ¯ p scat-tering, giving access to rare processes with crosssections below 1 pb. They are therefore used topursue a rich variety of searches for new phenom-ena.
2. Search for new phenomena in inclusiveDIS
Neutral Current Deep Inelastic Scattering (NCDIS) is measured at HERA for values of the pho-ton virtuality Q up to about 40000 GeV . Atthe highest Q , a good agreement between thedata and the SM expectation, derived from theDGLAP evolution of parton density functions de-termined at lower Q , is observed. Stringent con-straints on new physics can thus be set. For ex-ample, a finite quark radius would reduce thehigh Q DIS cross section with respect to theSM predictions. Using 1994–2005 data with anintegrated luminosity of 274 pb − , the ZEUS ∗ On behalf of the H1 and ZEUS Collaborations.
Collaboration ruled out quark radii larger than0 . × − m, assuming that the electron ispoint-like [1]. New interactions between electronsand quarks with caracteristic mass scale in theTeV range may also modify the cross section athigh Q via virtual effects. Many such interac-tions can be modelled as a four-fermion contactinteraction. Currently, the ZEUS data constrainthe effective scale Λ of eeqq contact interactionsto be larger than 2 to 7 . RRRLLRLLU6U5U4U3U2U1X6X5X4X3X2X1VAAAVV -0.2 -0.1 0 0.1 0.2 ± L (TeV -2 ) ZEUS ± p 274 pb -1 -1/ L best fit value +1/ L best fit valueallowed ± L range L - (TeV) L + (TeV) Figure 1. Limits at 95% C.L. on Λ for various con-tact interaction scenario from recent ZEUS data.1
E. Sauvan
3. Model dependent searches3.1. Leptoquarks
An intriguing characteristic of the StandardModel is the observed symmetry between the lep-ton and quark sectors. This could be a possibleindication of a new symmetry between the leptonand quark sectors, leading to “lepto-quark” in-teractions. Leptoquarks (LQs) are new scalar orvector color-triplet bosons, carrying a fractionalelectromagnetic charge and both a baryon and alepton number. Several types of LQs might exist,differing in their quantum numbers. A classifi-cation of LQs has been proposed by Buchm¨uller,R¨uckl and Wyler (BRW) [2] under the assump-tions that LQs have pure chiral couplings to SMfermions of a given family. The interaction of theLQ with a lepton-quark pair is of Yukawa or vec-tor nature and is parametrised by a coupling λ .
200 250 300 350 400 -2 -1 / GeV LQ M l S H1 (05) prelim. single LQH1 (94-00) single LQD0 pair prod.L3 indir. limit
200 250 300 350 400 -2 -1 e x c l u d e d Figure 2. Exclusion limits at 95% C.L. on thecoupling λ as a function of the leptoquark massfor S ,L in the BRW model. The indirect limitfrom L3 [4] and the direct D0 limits [5] are alsoshown. For comparison, the published H1 limiton S ,L using HERA I data [6] is also displayed.A new analysis has been performed by the H1experiment using 92 pb − of data recorded in2005 in e − p collision mode [3]. This large setof e − p data gives an increased the sensitivity toleptoquarks with fermion number F = 2. Forleptoquark couplings of electromagnetic strength( λ / π = α em ), F = 2 leptoquarks with masses up to 276–304 GeV are now ruled out, dependingon the LQ type. As an example, the new con-straint on S ,L leptoquarks are presented in Fig-ure 2 and compared to existing limits obtained atLEP by the L3 Collaboration [4] or at the Teva-tron by the D0 experiment [5]. The observed replication of three fermion fam-ilies motivates the possibility of a new scale ofmatter yet unobserved. An unambiguous signa-ture for a new scale of matter would be the directobservation of excited states of fermions ( f ∗ ), viatheir decay into a gauge boson and a fermion.Effective models describing the interaction of ex-cited fermions with standard matter have beenproposed [7,8,9]. In the models [7,8] the interac-tion of an f ∗ with a gauge boson is described bya magnetic coupling proportional to 1 / Λ whereΛ is a new scale. Proportionality constants f , f ′ and f s result in different couplings to U (1), SU (2)and SU (3) gauge bosons.The H1 experiment has carried out searchesfor both excited neutrinos and electrons using alldata recorded at √ s = 320 GeV. The total lumi-nosity analysed amount to up to 435 pb − [10].The new bounds on the ν ∗ and e ∗ masses obtainedas a function of f / Λ are presented in Figure 3(a)and (b), under the assumptions f = − f ′ and f = + f ′ , respectively. Assuming f / Λ = 1 /M ν ∗ and f = − f ′ , masses below 211 GeV are ruledout for ν ∗ . Excited electrons of mass below 273GeV are excluded at 95% C.L. if we assume f / Λ = 1 /M e ∗ and f = + f ′ . As observed in Fig-ure 3, the H1 analysis has probed new parameterspace regions and limits set extend at high massesprevious bounds reached at LEP and Tevatroncolliders.
4. Search for deviations from the SM inrare processes4.1. Events with high P T isolated leptons The production of a W boson in ep collisionsat HERA has a cross-section of about 1 pb. Theleptonic decay of the W leads to events with anisolated high transverse momentum lepton (elec-tron, muon or tau) and missing total transverse earches for New Physics at HERA P XT >
25 GeV obs./exp. obs./exp. obs./exp.H1 e − p
184 pb − . ± . . ± . . ± . e − p
204 pb − . ± . . ± . . ± . e + p
294 pb −
11 / 4 . ± . . ± . . ± . e + p
228 pb − . ± . . ± . . ± . P XT >
25 GeV byH1 and ZEUS experiments with SM predictions. * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -3 -2 -1 * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -3 -2 -1
101 ) -1 p, 184 pb - * HERA I+II (e n Search for f = - f’
LEP (L3)) -1 H1 HERA I (15 pb H1 *) n = 1/(M L f / H1 HERA I+II) -1 (184 pb(preliminary) (a) e* Mass [ GeV ]
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -4 -3 -2 -1 e* Mass [ GeV ]
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -4 -3 -2 -1 e* Mass [ GeV ]
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -4 -3 -2 -1
10 ) -1 s = 320 GeV, 435 pb (cid:214) Search for e*, HERA I+II ( f = + f’ H1 LEP (direct)LEP (indirect)) -1 H1 HERA I (120 pb ) -1 H1 HERA I+II (435 pb(preliminary)) -1 CDF Run II (202 pb = 1/(Me*) L f / (b) Figure 3. Exclusion limits on the coupling f /
Λ at95% C.L. as a function of the mass of the excitedneutrino (a) and electron (b) with the assump-tions f = − f ′ and f = + f ′ , respectively. Thenew limits set by H1 are represented by the yel-low area. Values of the couplings above the curvesare excluded. momentum. Of particular interest are events witha hadronic system of large transverse momen-tum ( P XT ). An abnormally large rate of high P XT events is observed by the H1 experiment [11] inthe electron an muon channels. In the analysisof all HERA I and HERA II data sets, whichamounts to a total luminosity of 478 pb − , 24events are observed at P XT >
25 GeV for a SMexpectation of 15 . ± .
5. Amongst them only3 events are observed in e − p collisions, in agree-ment with the SM expectation, while 21 eventsare observed in the e + p data for an expectationof 8 . ± . e + p and e − p data sets isexemplified in Figure 4 where the P XT distribu-tions of both data sets are displayed.The ZEUS experiment has carried out a similaranalysis using 432 pb − of 1996–2006 data [13].The results are also shown in Table 1. At P XT >
25 GeV the number of data events observed byZEUS is in agreement with the SM expectationin both e + p and e − p . A detailed comparison be-tween efficiencies of the H1 and ZEUS detectorsfor the W signal was performed. Both efficien-cies are comparable in the central region, whileH1 detection region extends to lower polar anglethan ZEUS. Nevertheless most of the high P XT events observed by H1 are lie within the range ofthe ZEUS acceptance.The analysis of the tau decay channel is alsoperformed by H1 on all HERA data with a totalluminosity of 471 pb − . In this channel, the sep-aration of the W signal from other SM processesis more difficult and the purity and efficiency arelower than for the e and µ channels. In total 20data events are observed compared to a SM ex- E. Sauvan pectation of 19 . ± .
2. One of the data eventshas P XT above 25 GeV, compared to a SM expec-tation of 0 . ± . channels m (GeV) e and XT P E ve n t s -1 channels m (GeV) e and XT P E ve n t s -1 H1 Data (prelim.)All SMSignal = 41
Data
N 4.8 – = 34.5 SM N ) -1 p, 294 pb + events at HERA I+II (e missT l+P (a) channels m (GeV) e and XT P E ve n t s -1 channels m (GeV) e and XT P E ve n t s -1 H1 Data (prelim.)All SMSignal = 18
Data
N 3.4 – = 24.4 SM N ) -1 p, 184 pb - events at HERA I+II (e missT l+P (b) Figure 4. Hadronic transverse momentum distri-bution of isolated lepton events observed by H1 in e + p (a) and e − p (b) data samples. The total SMexpectation is represented by the open histogramsand the contribution from W production by thehatched histogram. Multi-lepton production has also been studiedat HERA. Here the main production mechanismis photon-photon collisions. All event topologieswith high P T electrons and muons have been in-vestigated by the H1 experiment using a total lu-minosity of 459 pb − [14]. The measured yields ofdi-lepton and tri-lepton events are in good agree-ment with the SM prediction, except in the tailof the distribution of the scalar sum of P T of the leptons ( P P T ). In e + p collisions, 4 data eventsare observed with P P T >
100 GeV comparedto a SM prediction of 1 . ± .
2. No such eventsare observed in e − p collisions for a similar SMexpectation of 0 . ± . ee and eee topologies is also car-ried out by ZEUS using 446 pb − of data [15]. Inthis analysis, the SM contribution at high invari-ant mass in the di-electron channel is dominatedby Compton background due to a more difficult e/γ separation in the ZEUS detector. Neverthe-less 5 and 1 events with a high invariant massabove 100 GeV are observed in ee and eee chan-nels, respectively. This observation is in goodagreement with the corresponding SM expecta-tions of 3 . +0 . − . and 1 . +0 . − . , respectively. In a more general way, a broad range signa-ture based search has been developed by the H1Collaboration [16]. The same analysis is nowapplied to all HERA II data [17] and all finalstates containing at least two objects ( e , µ , j , γ , ν ) with P T >
20 GeV in the polar angle range10 ◦ < θ < ◦ have been investigated. The ob-served and predicted yields in each channel arepresented in Figure 5(a) and (b) for e + p and e − p collisions, respectively. The good agreement ob-served between data and SM prediction demon-strates the good understanding of the detectorand of the contributions of the SM backgrounds.A systematic scan of M all and P P T distributionsin each channel has been performed to look forregions of largest deviations to the SM. A statis-tical analysis is then used to quantify the signifi-cance of observed deviations. The largest devia-tion is observed in e + p data in the µ - j - ν channelwhich corresponds to the topology of isolated lep-ton events.
5. Conclusions
The most recent results of searches for newphysics performed at the HERA ep collider havebeen presented. Most of the analyses fully ex-ploit the complete data sample available whichamounts to ∼ . − per experiment. HERAappears to be very well suited to search for new earches for New Physics at HERA j-je-j-j m n j- n e-e-e m e- m - m g j- g e-j-j-je-j-j n j-j- n e-j- n -j- m g j-j- g e-j-e-j-j-j n j-j-j-j-j-j-j Events - -
10 1 10 j-je-j-j m n j- n e-e-e m e- m - m g j- g e-j-j-je-j-j n j-j- n e-j- n -j- m g j-j- g e-j-e-j-j-j n j-j-j-j-j-j-j Events - -
10 1 10 j-je-j-j m n j- n e-e-e m e- m - m g j- g e-j-j-je-j-j n j-j- n e-j- n -j- m g j-j- g e-j-e-j-j-j n j-j-j-j-j-j-j Events - -
10 1 10 SMH1 Data (prelim.) -2 -1
10 1 10 -1 p (178 pb + H1 General Search, HERA II e (a) j-je-j-j m n j- n e-e-e m e- m - m g j- g e-j-j-je-j-j n j-j-e-e-je-e-e n e-j- n -j- m g e-j- g - n j-e-j-j-j n j-j-j-j-j-j-j n j-j-j-j- Events - -
10 1 10 j-je-j-j m n j- n e-e-e m e- m - m g j- g e-j-j-je-j-j n j-j-e-e-je-e-e n e-j- n -j- m g e-j- g - n j-e-j-j-j n j-j-j-j-j-j-j n j-j-j-j- Events - -
10 1 10 j-je-j-j m n j- n e-e-e m e- m - m g j- g e-j-j-je-j-j n j-j-e-e-je-e-e n e-j- n -j- m g e-j- g - n j-e-j-j-j n j-j-j-j-j-j-j n j-j-j-j- Events - -
10 1 10 SMH1 Data (prelim.) -2 -1
10 1 10 -1 p (159 pb - H1 General Search, HERA II e (b)
Figure 5. The data and the SM expectationin event classes investigated by the H1 generalsearch. All channels with observed data eventsor a SM expectation greater than one event aredisplayed. The results are presented separatelyfor e + p (a) and e − p (b) collision modes. phenomena in specific cases, complementary tostringent bounds set at LEP and the Tevatron.Nevertheless, no convincing evidence for the ex-istence of new phenomena has been observed sofar. Among all event topologies investigated, thelargest deviation to the SM expectation is ob-served by the H1 experiment for isolated leptonevents in e + p collisions only. After having anal-ysed all data recorded by H1, this deviation cor-responds to a 3 σ excess of atypical W -like events. REFERENCES
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